1. Field of the Invention
The present invention relates generally to wireless communication, and more particularly to mobile to mobile wireless communications connections having improved voice quality.
2. Description of the Related Art
Wireless telephone communication systems utilize encoding to limit the amount of bandwidth required to transmit voice information over a communication channel. Instead of sending the received voice over the channel, a voice coder (vocoder) is used to first analyze the received voice signal and then generate a set of parameters which are used to drive a model that synthesizes a replica of the voice. It is these parameters that are transmitted at very low rates over the telephone network. Typically, in one type of vocoder, the voice is analyzed on a short time basis and a set of model parameters which may represent pitch, line spectrum frequencies (the frequency response of the vocal tract at any given time) and voice gain are generated.
In a digital wireless network, a vocoder is used at the transmitter to divide the voice signal of the person talking into a stream of fixed length frames of typically 20 msec. The vocoder then models the voice using a specific mathematical model and for each 20 msec frame, the vocoder generates a set of parameters for that mathematical model that best describes the voice for that 20 msec interval. Thus, the vocoder encodes the voice of the speaker into 20 msec frames. Each one of the frames carries a set parameter that represents the speaker's voice for the corresponding 20 msec interval. A modem at the transmitter transmits these frames over the air using an air interface technique such as CDMA, TDMA, or GSM. The modem function at the receiver passes the frames to a decoder at the receiver. The decoder decodes the 20 msec frames to produce the voice of the subscriber. A wireless subscriber uses a wireless terminal to access the wireless network.
The audio path from a mobile station to a desktop phone consists of a voice encoder at the mobile station, a wireless transmitter at the mobile station, a wireless receiver at the base station, and a decoder at the base station. There is one voice encoder and one decoder involved in the processing of the audio signal. This process is referred to as being a single vocoding operation because it involves a single encoding/decoding process.
The audio path from a first mobile station to a second mobile station consists of a voice encoder and a wireless transmitter at the first mobile station, and a wireless receiver and a voice decoder at the base station (or at the switch). The voice signal of the subscriber generated by the decoder is transmitted over the carrier's land lines to a second voice encoder at the second base station (or at the switch), and then to a wireless transmitter. A wireless receiver at the second mobile station receives the encoded voice signal and processes it in a second decoder to generate a voice signal. In this case, from the voice processing perspective, there is one voice encoder at the first mobile station, one decoder at the base station, another voice encoder at the second base station and a second decoder at the receiving mobile station. Thus, when a call is made from one mobile station to another mobile station, the voice signal is processed by two voice encoders and two decoders. This process is referred to as tandem vocoding processing because the voice signal is encoded twice and decoded twice.
The processing of the voice signal (the input speech) by the vocoding process (the process of encoding and decoding the voice signal) introduces a certain amount of digital distortion into the voice generated at the receiver relative to the actual voice uttered by the speaker. While, the quality of the voice generated by the decoder in single vocoding processing is very acceptable, it is somewhat below the voice quality of the input speech. Tandem vocoding compounds the digital distortion introduced into the voice signal and, therefore, it has been observed that the subjective voice quality of a mobile to mobile call is noticeably below that of a mobile to desk phone call. Thus, there exists a need to provide a method and apparatus for enhancing the voice quality of a mobile to mobile call.